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• W577531431 abstract "This thesis presents fundamental research in the field of laser vibrometry for theapplication to vibration measurements. A key concern for laser vibrometry is the effect oflaser speckle which appears when a coherent laser beam scatters from an optically roughsurface. The laser vibrometer is sensitive to changes in laser speckle which result fromsurface motions not in the direction of the incident beam. This adds speckle noise to thevibrometer output which can be indistinguishable from the genuine surface vibrations.This has been termed ‘pseudo-vibration’ and requires careful data interpretation by thevibration engineer. This research has discovered that measurements from smoothsurfaces, even when no identifiable speckle pattern is generated, can produce noise andtherefore reference to speckle noise, in such circumstances, is inappropriate. This thesishas, therefore, adopted the more general term of pseudo-vibration to include noisegenerated from any surface roughness or treatment, i.e. including but not limited tospeckle noise.This thesis develops and implements novel experimental methods to quantify pseudovibrationsensitivities (transverse, tilt and rotation sensitivity) with attention focussed oncommercially available laser vibrometers and consideration is given to a range of surfaceroughnesses and treatments. It investigates, experimentally, the fundamental behaviour ofspeckles and attempts to formulate, for the first time, a relationship between changes inintensity to pseudo-vibration sensitivity levels. The thesis also develops and implementsmodels for computational simulation of pseudo-vibrations using the fundamentalbehaviour of speckles. The combination of experimentation and simulation improvescurrent understanding of the pseudo-vibration mechanisms and provides the vibrationengineer with a valuable resource to improve data interpretation.Two experimental methods of quantifying pseudo-vibration sensitivity are developed andsuccessfully applied in the evaluation of transverse, tilt and rotation sensitivity for twomodels of commercial laser vibrometer. These evaluations cover both single beam(translational vibration measurement) and parallel beam (for angular vibrationmeasurement) modes. The first method presented requires correction of the vibrometermeasurement with an independent measurement of genuine velocity to produce aniiiapparent velocity dominated by the required noise components. The second methodrequires a differential measurement using two vibrometers to cancel common componentssuch as genuine velocity, leaving only uncorrelated noise from each measurement in theresulting apparent velocity. In each case, a third measurement is required of the surfacemotion component causing pseudo-vibration and this is used to normalise the apparentvelocity. Pseudo-vibration sensitivity is then presented as a map showing the spectralshape of the noise, as a mean and standard deviation of harmonic peaks in the map and asa total rms level across a defined bandwidth.The simulations employ a novel and effective approach to modelling speckle evolution.Transverse and tilt sensitivity are predicted for the first time and are verified by theexperimental study. They provide the vibration engineer with the potential to estimatepseudo-vibrations using a simple piece of software.The laser beam spot diameter has a large influence on the pseudo-vibration sensitivity.Transverse sensitivity has been quantified as around 0.03% and 0.01% (per order) of thetransverse velocity of the surface for beam spot diameters of 100 μm and 600 μmrespectively. Larger beam spots have been shown to significantly reduce transversesensitivity and measurements from smoother surfaces have also shown a reduced level oftransverse sensitivity. Tilt sensitivity has been quantified at about 0.1 μms-1/degs-1 and 0.3μms-1/degs-1 (per order) of angular velocity of the surface for beam spot diameters of 100μm and 600 μm respectively. Smaller beam spot diameters significantly reduce tiltsensitivity. The surface roughness or treatment has been shown to have little effect on thelevel of tilt sensitivity. Rotation sensitivity has been quantified at approximately 0.6 μms-1/rads-1 and 1.9μms-1/rads-1 (per order) of rotation velocity of the rotor for 90 μm and 520μm. Smaller beam spot diameters have shown a significant reduction in rotationsensitivity and measurements on smoother surfaces have shown a reduced rotationsensitivity. Focussing the laser beam approximately on the rotation axis has also shown asignificant reduction in rotation sensitivity. Parallel beam rotation sensitivity has beenquantified at 0.016 degs-1/rads-1 and it is demonstrated that this can adequately beestimated using the single beam rotation sensitivity." @default.
• W577531431 created "2016-06-24" @default.
• W577531431 creator A5077802503 @default.
• W577531431 date "2010-01-01" @default.
• W577531431 modified "2023-09-27" @default.
• W577531431 title "Uncertainty due to speckle noise in laser vibrometry" @default.
• W577531431 hasPublicationYear "2010" @default.
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